The number of hurricanes with sustained winds in excess of 131
mph - the catastrophic Category 4 and 5 strengths that Katrina
reached at her peak in the Gulf of Mexico last month - has nearly
doubled around the globe in the last 35 years, according to a
new study.

The study is certain to add
to the debate over whether global warming is fueling the stronger
hurricanes.

The researchers from the Georgia
Institute of Technology and the National Center for Atmospheric
Research looked at the strength of tropical storms in all the
world's oceans from 1970 through 2004 and found that the number
of Category 4 and 5 storms increased from about 11 a year in
the 1970s to 18 annually since 1990, according to their report
published Friday in the journal Science.

There are about 90 hurricanes
- called typhoons in the western Pacific and cyclones around
the Indian Ocean - across the globe every year.

There has been no overall increase
in the number of tropical storms over the past three decades,
but "Category 4 and 5 hurricanes are making up a larger
share of the total number of hurricanes,'' said Judith Curry,
head of atmospheric sciences at Georgia Tech and a co-author
of the study. She said the strongest storms "made up about
20 percent of all hurricanes in the 1970s, but over the last
decade they account for about 35 percent of these storms."

Peter Webster, a Georgia Tech
professor who led the study, argues that warming sea surface
temperatures - an average of 1 degree Fahrenheit worldwide as
a result of global greenhouse warming - is providing the water
evaporation that fuels hurricanes and can make them more powerful.

Separately, the National Oceanic
and Atmospheric Administration declared Thursday that Katrina,
while not the strongest hurricane to ever make landfall in the
United States, was the most destructive when it hit the central
Gulf Coast Aug. 29.

Although Katrina's intensity
at landfall was about 140 mph, weaker than the 160-mph winds
assigned Hurricane Camille when it hit the same area in 1969,
officials said Katrina was much more destructive because the
storm's hurricane force winds extended over a wider area - about
120 miles from its center.

Katrina was fueled by water
2 to 3 degrees warmer than normal in the Gulf, and hurricane
experts universally agree that warmer-than-normal sea temperatures
have prevailed in the hurricane-breeding waters of the North
Atlantic and the Caribbean since the mid-1990s.

But there's debate about whether
global warming has really had so much effect on hurricanes. Kerry
Emanuel, a meteorologist at the Massachusetts Institute of Technology,
who published a paper reaching similar conclusions on global
hurricane intensity last month in the journal Nature, noted that
"the upswing has been attributed by many researchers to
global warming."

Others, like James O'Brien
at Florida State University, say there's no evidence that areas
of warm water in the tropical oceans are increasing in size outside
of natural cycles.

Chris Landsea, a leading hurricane
researcher now overseeing scientific operations at the National
Hurricane Center in Miami, said that while the new study shows
that the percentage of stronger storms is up globally, neither
the duration of the storms nor their average maximum wind speeds
have increased. "That's not physically consistent with more
intense storms, there's something fishy in that result,'' he
said.

Landsea doesn't think the researchers
did bad science, but may have relied on poor data - estimates
of maximum wind speeds of hurricanes obtained by contrasting
the temperature of a storm's eye and that of adjacent cloud tops
as displayed in satellite images.

This technique wasn't developed
by NOAA scientist Vernon Dvorak until 1973 and wasn't put into
use worldwide until the 1980s, "so there was no way for
even crude estimates of strength for storms outside the Atlantic
and some in the western Pacific until the middle of the study
period,'' Landsea said.

Only NOAA and the U.S. Air
Force regularly fly reconnaissance aircraft into Atlantic hurricanes
to measure wind speed, barometric pressure and other aspects.
This information has been used to improve the satellite estimates
over the last two decades.

Landsea said the fact that
the study showed only a 5 percent increase in number of intense
storms in the Atlantic during the past 15 years - during a period
when storm activity picked up as part of a natural multi-decadal
cycle - suggests that the storm data from other oceans may not
be so reliable.

Greg Holland, a researcher
at NCAR in Boulder, Colo., and a co-author of the study, agreed
that there have been improvements in intensity measurements since
the 1970s, and that the 30-year period might not capture some
still unknown cycles of hurricane activity around the world.
"But there is no doubt that there is a substantial increase
in intensity here,'' he said.